This invention relates to an integrated system for converting oxygenates, such as methanol or dimethyl ether (DME), to liquid hydrocarbons. In particular it provides a continuous process for producing distillate range fuel products by dehydrating the oxygenate feedstock catalytically to produce an intermediate lower olefinic stream, oligomerizing the olefins to produce distillate/gasoline, and hydrogenating the distillate product to provide a stabilized product for use as diesel fuel or the like.
Recent developments in zeolite catalysts and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks, for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalysts, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. This process may supplant conventional alkylation units. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the reactor system for further conversion to distillate-range products. Operating details for typical MOGD units are disclosed in copending U.S. Pat. No. 4,456,779 and U.S. Pat. No. 4,433,185, incorporated herein by reference.
In addition to their use as shape selective oligomerization catalysts, the medium pore ZSM-5 type catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Particular interest has been directed to a catalytic process for converting low cost methanol to valuable hydrocarbons rich in ethene and C.sub.3.sup.+ alkenes. Various processes are described in U.S. Pat. Nos. 3,894,107 (Butter et al), 3,928,493 (Chang et al), 4,025,571 (Lago), and in copending U.S. patent application Ser. No. 388,768, filed June 15, 1982 (Yurchak et al) and now abandoned. Significance of the methanol-to-olefins ("MTO") type processes, especially for producing ethene, is discussed in Hydrocarbon Processing, November 1982, pp. 117-120. It is generally known that the MTO process can be optimized to produce a major fraction of C.sub.2 -C.sub.4 olefins; however, a significant C.sub.5.sup.+ byproduct is coproduced, including polymethylbenzenes, such as durene, as described in U.S. Pat. No. 4,025,576 (Chang et al). Prior process proposals have included a separation section to recover ethene and other gases from byproduct water and C.sub.5.sup.+ hydrocarbon liquids. Treatment of the C.sub.5.sup.+ liquids to dealkylate the polymethylbenzenes has been necessary to convert this fraction to satisfactory liquid fuel, for instance as disclosed in U.S. Pat. No. 4,347,397 (Dwyer et al) and U.S. Pat. No. 4,387,261 (Chester et al). Such post treatment processes add significantly to the cost of liquid fuels plant.